The present invention relates to a method of reducing the NO.sub.x -emissions during combustion of nitrogen-containing fuels via burners in a closed combustion chamber; fuel and air for combustion are supplied to the burner flame in stages as partial flows via delivery means which are separate from one another.
The reaction modes which cause the formation of nitrogen oxides in industrial firing equipment are largely known.
A method of the above mentioned type is known from the U.S. Pat. No. 4,395,223. In FIGS. 9 and 10 of this Patent Letter a large-sized box furnace is shown, in the one vertical wall of which a row of primary burners for burnung a liquid fuel together with combustion air injected around the liquid fuel in a primary flame zone without internal back flow at a rate in excess of the stoichiometric rate required for the combustion of the liquid fuel is provided, i.e. the fuel is burnt in a first stage combustion zone under fuel lean conditions. In the wall above the row of primary burners there is arranged a row of secondary or reduction fuel nozzles or burners, namely two secondary fuel nozzles for each primary burner, which lead to the generation of a second stage combustion zone operated under fuel rich conditions.
Downstream the reduction fuel nozzles there is provided a row of air nozzles for generating a third stage combustion zone downstream the secondary flame zone supplied with stage air at a rate not less than the stoichiometric rate required for the final burn out of the unburnt component. In case of the shown box furnace the reduction fuel is injected toward a location downstream of the combustion gas of the primary flame zone and in the specification it is also mentioned that the reduction fuel might be injected around the primary flame zone.
In another known method (FIG. 5 of U.S. Pat. No. 4,4O3,941) there is provided a combination of a primary burner with a secondary burner or reduction fuel supply in at least two stages so that a primary flame zone and a secondary flame zone are repeatedly formed in the direction of the upward gas stream in the furnace, but independant from the number of stages upstream there is only one final combustion zone generated downstream.
Since there is only one final combustion zone for a plurality of primary and secondary burner combinations the NO.sub.x -emission control under load variations is less effective, especially if one row of primary burners and associated reduction fuel supplies are shut off and partial load of the combustion process. Further, in the area of primary and secondary flame zones repeatedly formed along the wall, there is a oxygen-lean atmosphere leading to corrosion of the wall and slagging on the wall.
It is an object of the present invention, when burning nitrogen-containing coal dust via a plurality of primary burners one arranged above the other, to secure sufficent reduction of NO.sub.x -emission under varying load conditions.
It is another object to keep the oxygen-lean areas along the wall as small as possible.